The invention relates generally to belts used in different phases of a papermaking process. More particularly, the invention relates to shoe press belts used in shoe presses of press sections in board, paper and pulp machines, as well as in shoe presses of a paper machine calender when manufacturing certain types of paper.
Shoe presses are commonly used in pulp, cardboard and paper machines for dewatering a fibre web. One surface of a wet fibre web travelling at a high speed in such shoe presses is pressed by a rotating press roll while simultaneously the other surface of the fibre web is pressed by a stationary pressure shoe surrounded by an endless belt loop having an elastic elastomer body. The belt travels between the press roll and the pressure shoe at approximately the same speed as the fibre web. The fibre web on top of a press felt is driven through the shoe press with the outer surface of the press belt in direct contact with the press felt and the inner surface of the press belt gliding against the pressure shoe. Lubricating oil is typically provided between the belt and the pressure shoe to guarantee free movement of the belt.
Prior art press belts typically contain an elastomer material, such as a polyurethane or rubber, and a support structure made of yarn inside the belt. An example of a shoe press belt is disclosed in patent application FI 20040166.
Generally speaking a shoe press belt is subjected to recurrent high flexing and pressing forces between a press roll and a pressure shoe as the fibre web travels at a high speed through the shoe presses. In the course of time such strains may cause the belt material to crack during use, which will eventually damage the belt beyond usability. A general trend has been to increase the web speed in paper machines to improve manufacturing efficiency. Since the same dry matter content as before is aimed at with the higher speed, pressing force must be increased. Hence also shoe press belts used in the pressing steps are subjected to increasingly higher loads, the belts being thus required to possess a plurality of various properties in order to ensure high performance. As web speeds in paper machines increase, the belt is subjected to an ever higher thermal load due to the increased speed, on the one hand, and the warming of the lubricating oil on the other hand. A good heat resistance of the belt is an essential aspect in view of the service life of the belt.
The operating temperatures of shoe presses in paper, board and pulp machines are usually below 70° C. Lately it has been observed that manufacturing efficiency can be increased also by raising the temperature of the shoe presses; higher fibre web temperature may allow a better dry matter content to be achieved as water viscosity decreases and water exits more efficiently. The fibre web may be heated by means of steam, a hot roll or by raising the temperature of the mass, for example. In what is known as an impulse drying technique the temperature is raised to as high as over 200° C. Also in what are known as shoe calenders the temperature of the hot roll may be over 200° C. The same factors raise the temperature of the shoe press belt from the outside. Increasing of the pressure load improves the dry matter. At the same time, however, heat caused by friction heats the oil lubricating the shoe and raises temperature inside the shoe press belt. These factors together increase the heat resistance requirements directed to the shoe press felt.
Shoe press belts made of ordinary polyurethane sustain well temperatures below 70° C. However, at higher temperatures the mechanical and dynamical characteristics of polyurethane and, correspondingly, those of the shoe press belt deteriorate clearly. Polyurethane softens at a high temperature and the wearing of the belt increases. Due to the softening the belt grooves become compressed and do not return to their original shape, which impairs dewatering because the grooves are no longer able take enough water. At a high temperature, and particularly in the presence of residues of paper manufacturing chemicals, thermal and chemical oxidation reactions start to take place in the polyurethane, causing polymer chains to break. This breaking of the chains in turn deteriorates the properties of the polyurethane, which then seen as wearing of the belt and as weakening of break resistance. At its location of use, the shoe press belt is under constant dynamic pressing and tensile load. This is why it is important that its dynamic properties remain good in this temperature range.
Belts suitable for shoe presses have been described abundantly in the art. EP 1338696 A1, for example, discloses a belt, such as a shoe press belt, suitable for the pressing steps in a papermaking process, the belt comprising a reinforcing substrate embedded in a polyurethane layer. The outer peripheral surface of the belt, which is in direct contact with a press felt supporting the fibre web, is formed of a urethane prepolymer and dimethylthiotoluenediamine (DMTDA) as a hardener. The belt aims at decreasing or slowing down cracking, if any, which generally occurs on outer surfaces of the belts in particular, as well as at preventing delamination between the hardener and the polyurethane layer.
EP 877118 A2 discloses a shoe press belt with a base layer and a resin layer on both sides thereof. The resin is made of a toluene diisocyanate polyether type urethane prepolymer, with 4,4′-methylene bis(2-chloroaniline) (MOCA/MBOCA) as a chain extender. The belt is mentioned to provide the required resistance to wear, flex fatigue strength and cracking prevention.
WO 2005/090429 A1 discloses a belt suitable for a shoe press, the belt comprising a urethane-based coating containing nanoparticles. The aim in adding nanoparticles to the coating is, for example, to improve the belt's resistance to flex fatigue and crack propagation, and to provide the belt with hardness and wear characteristics.
EP 0939162 A2 discloses a shoe press belt comprising two resin layers made of a toluene diisocyanate polyether type urethane prepolymer, with 3,3′-dichloro-4,4′-diaminodiphenylmethane (MOCA/MBOCA) as a reinforcing substrate. The aim is to obtain a belt providing strength both in machine and in cross machine direction and to prevent delamination of resin from the belt.
A problem with the above disclosed shoe press belts is that their heat resistance is not sufficient in the production rates ever increasingly preferred in board, paper and pulp machines for maximum manufacturing efficiency.
A solution disclosed in EP 1136618 A2 with the view of an increase in the temperature during the operation of a pressure belt is to use an insulating filler agent. Likewise, DE 19702138 A1 proposes to improve heat resistance of press belts by using heat conducting filler particles in an elastomer matrix. In DE 19651557 A1 a press belt is provided with a coating that protects the elastomer underneath both from chemicals and heat.
However, solutions of the above type, which are based on the use of filler agents or a coating, may cause other problems in the belts. The polymer material may become brittled or delaminate from the reinforcing substrate because of the filler agent. In long-term use of the belt in particular, problems may also arise in the adhesion of the coating to the belt material.
It is therefore desirable to obtain, in a yet simpler manner, shoe press belts which have a good heat resistance and are free of the prior art problems, and which maintain their good dynamic and mechanical properties in a wider operating temperature range and at higher temperatures in particular.